Astrophys. J., 705, L25-L30 (2009/November-1)
Bona fide, strong-variable galactic luminous blue variable stars are fast rotators: detection of a high rotational velocity in HR Carinae.
GROH J.H., DAMINELI A., HILLIER D.J., BARBA R., FERNANDEZ-LAJUS E., GAMEN R.C., MOISES A.P., SOLIVELLA G. and TEODORO M.
Abstract (from CDS):
We report optical observations of the luminous blue variable (LBV) HR Carinae which show that the star has reached a visual minimum phase in 2009. More importantly, we detected absorptions due to Si IV λλ4088-4116. To match their observed line profiles from 2009 May, a high rotational velocity of vrot≃ 150±20 km/s is needed (assuming an inclination angle of 30°), implying that HR Car rotates at ≃0.88±0.2 of its critical velocity for breakup (vcrit). Our results suggest that fast rotation is typical in all strong-variable, bona fide galactic LBVs, which present S-Dor-type variability. Strong-variable LBVs are located in a well-defined region of the HR diagram during visual minimum (the "LBV minimum instability strip"). We suggest this region corresponds to where vcrit is reached. To the left of this strip, a forbidden zone with vrot/vcrit>1 is present, explaining why no LBVs are detected in this zone. Since dormant/ex LBVs like P Cygni and HD 168625 have low vrot, we propose that LBVs can be separated into two groups: fast-rotating, strong-variable stars showing S-Dor cycles (such as AG Car and HR Car) and slow-rotating stars with much less variability (such as P Cygni and HD 168625). We speculate that supernova (SN) progenitors which had S-Dor cycles before exploding (such as in SN 2001ig, SN 2003bg, and SN 2005gj) could have been fast rotators. We suggest that the potential difficulty of fast-rotating Galactic LBVs to lose angular momentum is additional evidence that such stars could explode during the LBV phase.
stars: atmospheres - stars: individual: HR Carinae - stars: mass loss - stars: rotation - stars: variables: other - supergiants
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